1. Field of the Invention
The present invention relates to radiofrequency shielding, and more particularly, relates to a low impedance radiofrequency shielded window.
2. Description of Related Art
The reception and transmission of electromagnetic signals is important to the proper functioning of many devices. Cellular telephones and email devices, notebook computer wireless modems, Global Positioning System receivers, wireless data devices, and the like, all rely on the use of electromagnetic signals in the radiofrequency spectrum to operate. Similarly, many sensitive electronic devices may be susceptible to electromagnetic signals in general or radiofrequency signals in particular, and must be shielded from such signals when under test. Testing, tuning, and repair of such a device can require that the Device Under Test (DUT) be isolated from ambient electromagnetic radiation. Exposing a receiving device to an unambiguous test signal free from electromagnetic interference, or obtaining a corresponding clean signal from a transmitting device, is often necessary for testing and calibration. Typically, a device to be tested is placed inside an electromagnetic isolation (EMI) chamber, sometimes also known as a Faraday cage, which comprises a continuous shielded enclosure which prevents ingress or egress of electromagnetic radiation. Such a chamber may take the form of an entire room, known in the art as a “screen room” for its continuous screen shielding, wherein an operator testing a device may also be enclosed in the room with the device.
A screen room can be quite expensive and therefore may not be economically feasible for a small business. Likewise, a screen room can be quite large making it unsuitable for many test or operating environments. Smaller shielded enclosures, also known as shielded boxes, capable of housing a Device Under Test but not an operator, are disclosed, for example, in U.S. Pat. No. 4,884,171 to Maserang et al.; U.S. Pat. No. 5,136,119 to Leyland; U.S. Pat. No. 5,171,936 to Suzuki et al.; U.S. Pat. No. 5,594,200 to Ramsey and more recently U.S. Pat. No. 8,294,044 also to Ramsey. The entire disclosures of these patents are incorporated herein by reference.
While some radiofrequency shielded enclosures are made from a screen shielding that allows for some visibility of the device under test, many radiofrequency shielded enclosures are made from a solid material that necessitates the use of a shielded window to allow for viewing of the device under test and its related switches, controls, displays, and user interface.
Shielded windows to permit viewing of the device under test are disclosed, for example, in the '171 patent to Maserang et al. and the '200 patent to Ramsey. The entire disclosures of these patents are incorporated herein by reference. Typically, a metallic screening is placed against a glass or clear plastic window material, the metallic edge of this screening then makes electrical contact to the enclosure, thus ensuring the shielded window is at the same ground potential as the enclosure walls. To increase radiofrequency shielding effectiveness, two or more layers of metallic screening and glass or clear plastic window material are sometimes sandwiched together, with the layers of metallic screening daisy chained together in series before making electrical contact to the enclosure's (grounded) walls. At lower frequencies, the resistance of each layer to ground is very low and not dominant, but at higher radio frequencies, the inductance of the ground path becomes vitally important to effective shielding. Typical inductance of a layer can be 5-10 nanohenries, and at a frequency of 2 Ghz., this equates to 10-20 ohms to ground. In addition, with most woven metallic screen shielding, the contact points of each screening element are subject to oxidation and less than optimal contact, thus greatly reducing the shielding effectiveness of the shielded window across its area.
What is needed is a radiofrequency shielded window with a low impedance path to ground, even at higher frequencies. What is also needed is a radiofrequency shielded window where the screening is not subject to impedance degradation at the contact points. What is further needed is a radiofrequency shielded window that is lower cost and more robust than currently available shielded windows.
It is therefore an object of the present invention to provide a low impedance radiofrequency shielded window.
These and other objects of the present invention are described in the detailed specification, drawings and claims contained herein.